Planning a bike packing with a friend immediately after finals, meaning we have no time to train. Will be cycling for around a month through Europe, on road bikes.
I ride a lot but my friend hasn’t ridden a road bike before and is borrowing one just before we leave.
What is a reasonable daily distance to target? We are both in good shape and were thinking of 100km/day does this sound reasonable?
I can think of worse things to get a mate to do without training, but not many.
If you told me your mate ran a marathon last week, this answer would not change. It is not about how fit they are, it is about how bike fit they are.
Years ago (when I was mid in my 20's) I was driving home for work on a Friday afternoon. The radio advertised a 40/100km fun ride happening locally, sponsored by my LBS that was just a block ahead of me. I stopped, walked in to sign up for the 40, decided what the... and signed up for the 100. I had been off my bike for over 8 months, initially due injury, then got into running and the bike collected dust. Before that I had complete a few 100 milers on that bike. I had a good bike fit and was fit, but was not bike fit.
I did the 100km, in a respectable time considering, but it hurt. It hurt a lot more the next day, no way was I riding far (I did a very easy 10km to get things moving). You will likely have issues with saddle comfort, if the road bike is a racer, your friend may struggle with the length of time in the position needed. Given your mate is on a borrowed bike and not an experienced cyclist, the bike fit is likely to be poor.
What I would do is plan on a much lower milage for the first few days, building up over a couple of weeks. Allow for plenty of time off bike including full rest days and short days. Your first two weeks will need to have conservative, or very flexible, plans.
Most likely the biggest problem will be saddle soreness and chaffing. With no pre-conditioning, on a borrowed bike, 100km (4 hours in the saddle at 25km/h average) is likely to make things rather uncomfortable. If doing this every day, no time to recover.
Your friend really needs to get a bike (ideally the one they are borrowing) and make the time to put in a couple of back-to-back 80km+ rides. Ideally you join them and work out how you ride together (I expect you, as the stronger and more experienced rider, will take point much of the time). From the information gained, you can then work out a plan that has a greater chance of a successful trip. If your friend cannot put aside time for a couple of rides, you will need to assess your friend's tolerance for discomfort, and consider if it is a trip for another time.
In addition to the other good answers, I think you should ask yourself if you just want to be on the road, or that you want to enjoy other things as well.
I have been on several (camping) cycle holidays and, for me, a holiday is just that: A holiday, not a long distance race. But that's of course a personal choice, and others may enjoy cycling as far as they can.
To get some idea, a typical day for me would look like this: Wake up and eat breakfast, get a coffee, get another coffee, slowly start packing, leave between 10-11, have lunch after an hour or two, visit something, cycle two more hours, then start looking for a place to sleep, arriving around four, with plenty of time to have a drink in a bar, get groceries or find a place to eat.
OTOH, last year I met a guy who would travel 150-200 km per day, but he left at 8 in the morning and arrived at 8 at night, carrying only a minimal amount of gear.
I'm reasonably trained, and I can certainly ride 100 km in a day if necessary, but usually it's more like 60-80. And this is mostly on paved roads, without hills or mountains. It also depends on whether you will be camping, or staying in ho(s)tels, and how much stuff you will be carrying around.
The "reasonable distance" for such travel is the one that the least trained member of the team is sure to able to cover day after day, and there's no way to know it without training/preparation. It might be pedantic as answer, but planning something of this scale without knowing the most important parameter is an impossible exercise. If you'd like to make the best of this trip, the best is probably to lower uncertainties and training is the way to do it.
Note that a 2-hour ride for someone fit but not "bike fit" can be already painful, and may require some recovery time to ensure that pain the days after won't remove any enjoyment from the rest of the trip. Even for someone who bikes a lot, changing "bike style" requires an adaptation time, with potential recovery periods - different muscles are used, that need to be built up. And you also need to be sure that the bike fits you. While some adjustment can be done when traveling (if you know what to adjust), some are more difficult to change - bike size, saddle (although saddles can be easily swapped, so finding the right one can be an objective of the training/preparation).
There's also a "flag" with the keyword bikepacking: while not having to stick with a strict planning gives a lot of flexibility, it's also a potentially another source of problem: if you spend less time that foreseen on the bike, you'll need to find something else to do, and taking care that nothing happens to the bikes when you are doing your other visits.
